Iron oxide precipitation from acidic iron salt solutions

ABSTRACT

Improved methods for treating metallurgical compositions involve reacting a metallurgical composition with an aqueous nitric acid solution. The reaction is performed at a pressure or at least about 220 psig and at a temperature of at least 100° C. The metallurgical composition comprises iron and one or more non-ferrous metals. The reaction dissolves at least a portion of the non-ferrous metal compositions into the solution which is in contact with solid ferric oxide. The reaction can be repeated on the isolated solids to increase the purity of ferric oxide in the solids. Zinc can be removed from mixed metal solutions obtained from furnace dust by adding base to precipitate zinc hydroxide.

This application is a continuation-in-part of U.S. application Ser. No.10/834,522 filed Apr. 29, 2004 now U.S. Pat. No. 7,399,454, and claimsthe benefit thereof.

FIELD OF INVENTION

The present invention relates to hydrometallurgical chemistry. Moreparticularly, the invention relates to acid leaching of iron salts, andprecipitation of selected hematites from a solution of iron salts.

BACKGROUND OF THE INVENTION

Scrap metal can be recycled into quality steel using an electric arcfurnace (EAF). In an EAF, the scrap metal is melted with electric arcsformed to the scrap metal. The scrap metal can include small amounts ofnon-ferrous metal and the like. The EAF process operates as a batchmelting process, producing batches of molten steel. The EAF is a highlyeffective melting apparatus. A significant fraction of steel produced inthe U.S. is produced with an electric arc furnace.

However, a drawback in the EAF manufacture of steel is the production ofEAF metallurgical dust waste by-products. EAF dust is generated duringthe steel making process by a variety of mechanisms, including dropletejection from the turbulent melt and vaporization. The vaporizationmechanism is primarily responsible for the relatively high proportion ofthe non-ferrous metals in the dust such as zinc, lead, tin, chromium,copper and cadmium. The vaporized metals condense as oxides and ferritesand generally are collected downstream in a baghouse and/orelectrostatic precipitator. Due to the presence of non-ferrous metals inthe dust, the furnace dust cannot be directly recycled. The productionof 1 ton of steel can generate approximately 34 pounds (15.4 kg) ofwaste EAF metallurgical dust.

The rapid growth of the EAF steel process has made EAF metallurgicaldust one of the fastest growing and one of the most significantenvironmental problems worldwide. At present, there are approximately600,000 metric tons of EAF waste generated annually in the USA and anadditional 600,000 metric tons generated annually in the rest of theworld. There are also similar quantities of metallurgical dust at alower level of contamination that is derived from the other majorprocess for steel manufacturing, the Basic Oxygen Furnace (BOF). Becausethe levels of toxic metals such as cadmium, lead and zinc are lower inBOF metallurgical dust, BOF dust is not currently classified by the EPAas hazardous. However, BOF metallurgical dust has non-iron contaminantsthat make it difficult to utilize it in current steel manufacture. Thus,BOF metallurgical dust may end up as unused waste.

EAF metallurgical dust may contain high concentrations of iron(approximately 25%), zinc (approximately 25%), lead (approximately 5%),and smaller amounts of tin, cadmium, chromium and copper. The remainderof the dust is silica, lime and alumina. The nonferrous values representpotentially rich sources of metal values. Due to the presence ofpotentially hazardous metals, such as lead, chromium and cadmium, theEAF dust cannot be disposed in landfills since the hazardous metals mayleach out due to rain or underground water to contaminate neighboringwater sheds. Thus, the processing of the dust is an important commercialand environmental issue. Some specific examples of metal content forthree samples of EAF dust are presented in Table 1.

TABLE 1 SAMPLE PLANT EAF DUST CONSTITUENTS FOR THREE DIFFERENT SAMPLES.% Zn  % AI % Pb % Fe % Cd % Cu % Mn % Na % Ba 1 20.3 0.27 1.27 36.0 0.02016 3.54 0.59 0.01 2 22.7 0.30 1.04 34.8 0.01 0.13 3.60 0.70 0.01 3 27.0— 1.4 26.0 0.081 — 3.4 — — % CaO % Cr % Mg % Ni % V % As % SiO2 % CI 15.51 0.20 2.06 0.02 0.01 .0036 2.52 0.96 2 5.48 0.20 2.48 0.13 0.02.0029 4.74 0.78 3 — 0.25 — — — — — —

SUMMARY OF THE INVENTION

The invention provides a method and an apparatus for convertinghazardous metallurgical dust into manageable chemical products, such asnon-toxic waste and/or marketable chemical products. Methods describedherein can be based upon leaching of metallurgical dust at elevatedpressure and temperature with nitric acid and the recovery of ferricoxide. The leaching step can be repeated to further purify the resultingferric oxide solids. Also, one or more preliminary purification stepscan be performed. Zinc can be removed from mixed metal solutionsobtained from furnace dust by adding base to precipitate zinc hydroxide.

In a first aspect, the invention pertains to a method of reacting ametallurgical composition with an aqueous nitric acid solution. Thereaction is performed at a pressure of at least about 220 psig and at atemperature of at least 100° C. The metallurgical composition comprisesiron and one or more non-ferrous metals. The reaction dissolves at leasta portion of the non-ferrous metal compositions into the solution whichis in contact with solid ferric oxide.

In another aspect, the invention pertains to a method for precipitatingferric oxide comprising subjecting an aqueous ferric nitrate solution toa temperature of at least about 100° C. at a pressure of at least about220 psig.

In a further aspect, the invention pertains to an apparatus for treatingmetallurgical dust comprising a sealed pressure vessel at a pressure ofat least about 220 psig holding a mixture of a metallurgical compositioncomprising at least iron and a solution of nitric acid.

In an additional aspect, the invention pertains to a method forisolating zinc from a mixed metal solution obtained from furnace dust,the method comprising adding base to precipitate zinc hydroxide.

As outlined above, iron in solution at elevated temperature and pressurehydrolyzes and is precipitated as iron oxide. EAF dust is leached innitric acid at high temperatures and pressures and then re-precipitatesas solid ferric oxide. The non-iron metals from the EAF dust aredissolved in solution leaving a solid precipitate, containing iron asferric oxide. This process was demonstrated to be effective forseparating the iron from “non-iron” metals. However, insoluble compoundsin the dust (i.e., silicates) remain with the ferric oxide and may alterthe colour properties of the ferric oxide solids obtained by thisprocess.

Alternatively, it was unexpectedly found that modifications of theabove-described process resulted in the production of pigment gradeferric oxides. The process outlined above was modified to completelysolubilize the iron and other metals so that insoluble components couldbe removed from the system. The solution is subjected to hightemperature and pressure to precipitate pigment grade ferric oxides.

In one embodiment of the modified process, ferric oxide solidsprecipitate from an iron-containing metal nitrate solution subjected toelevated temperature and pressure. Precipitates obtained from thismodified process are black solids about 20 to 30 microns in diameter.X-ray diffraction analysis identifies these solids as hematite (ferricoxide) which is the same compound contained in synthetic red iron oxidepigments. When examined by scanning electron microscopy, the particlesof the black precipitates do not appear spherical (as occurs with rediron oxide pigments) and instead appear like grape clusters comprised ofmany smaller particles connected together. The different crystalstructure of the clustered particles causes light to reflect differentlymaking them appear dark and preventing their use as red iron oxidepigment.

Alternatively, a further modification of the already modified processcan be utilized. In another embodiment, a seed solid is added to theiron salt solution and subjected to elevated temperature and pressure toresult in the hydrolysis of iron to pigment grade ferric oxide solids.Pigment grade hematite is generally comprised of fine particles havingan average size of less than 2 microns and are red in colour. Seeding iscommonly used in the metallurgical fields to allow precipitationproducts to grow and become larger in size. Unexpectedly it was foundthat following the addition of ferric oxide (hematite) seed to a ironsalt solution, the precipitated particles are finer than the blackprecipitates obtained from unseeded reactions. It was also unexpectedlyfound that the more seed material used, the finer the precipitates were.The precipitates obtained by the seeded process are generally less than2 microns in size, although coarser products are possible. Scanningelectron micrographs of the precipitates from the seeded process revealthat they are generally spherical in nature. This is in contrast to theprecipitates comprising hematite particles obtained from the modifiedbut unseeded process. Unexpectedly, the seeded process results in theproduction of iron solid precipitates, iron oxides, that have size andcolour characteristics which make them desirable for use as syntheticiron oxide pigments.

In various embodiments, there is provided a process for the productionof ferric oxide precipitates having a selected particle size, comprisingselecting a combination of a temperature and a seeding ratio, andconducting said process at pressures above atmospheric to obtain ferricoxide precipitates of the selected particle size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart of the process for treating a metallurgicalcomposition, such as EAF dust, to produce a filtrate solution and asolid comprising ferric oxide.

FIG. 2 is an expanded flow chart of FIG. 1, which adds a second leachingstep to the process of obtaining ferric oxide.

FIG. 3 is schematic diagram depicting an apparatus to conduct a nitricacid leach of a metallurgical composition under pressure.

FIG. 4 is an x-ray diffractogram for one sample following a pressurizedleach treatment.

FIG. 5 is an x-ray diffractogram of another sample following apressurized leach treatment.

FIG. 6 is an x-ray diffractogram of an EAF dust sample prior to a nitricacid treatment.

FIG. 7 is a process flow diagram for the seeded precipitation process.

FIG. 8 illustrates the size and colour of the precipitates obtainedunder different reaction conditions of the seeded processes.

DETAILED DESCRIPTION

As described herein, an improved process for the treatment ofmetallurgical dust involves the leaching of the metallurgical dust withnitric acid at elevated pressure conditions, and generally elevatedtemperature conditions, to dissolve one or more of the metal componentsof the dust while leaving a residue of ferric oxide. This process isbuilt on the principle that at appropriate pressure and temperatureconditions, iron forms solid iron oxide, specifically ferric oxide, in anitric acid/nitrate solution. Since many of the other metal contaminantsare solublized in nitric acid under the conditions at which ferric oxideis a precipitate, the ferric oxide can be separated from the other metalconstituents. The high pressure leaching process can be repeated on theresidue/precipitate from one pressurized leaching step to obtain aimproved purity level of the ferric oxide. It has also been discoveredthat an initial low pressure leach can optionally be performed toinitially process the dust to remove significant quantities ofnon-ferrous metals under pH conditions at which the iron remains insolid form. The high pressure leach approaches can be used to recoveralmost all of the iron from the original dust in very pure forms.

Metallurgic dust, as used herein, is any unpurified metal compositioncomprising a significant proportion of iron compositions. Suitable metaldust includes, for example, metallurgical dust from steel manufacturingprocesses, such as EAF dust, as well as iron scrap, iron rouge recoveredfrom steel cleaning lines, mill scale, iron containing minerals and lowgrade iron based pigments. Since it is impure, the metallurgical dustgenerally comprises at least about 10 mole percent of non-ferrous metalsprior to processing.

In the improved processes herein, the formation of ferric oxide solidsreleases the corresponding nitric acid that at ambient pressureconditions would form a ferric nitrate solution. Better separation ofthe non-ferrous metals from the ferric oxide is accomplished with excessnitric acid present during the high pressure leach. In some embodiments,the excess nitric acid can be directly recovered for reuse withoutevaporating water since the nitric acid in the filtrate solution can bediverted back into the leaching process, possibly with the addition ofmore acid. Alternatively, the filtrate can be diverted to recover othermetal constituents in the solution. The high pressure leach processesdescribed herein can be used to obtain very high purities of ferricoxide in an efficient process with materials that can be readilyhandled.

A high proportion of the zinc in EAF metallurgical dust is present inthe form of ferrites (ZnO.Fe₂O₃), which have proven resistant toleaching processes. Some leaching techniques have used a two-stage leachunder ambient temperature and pressure in order to obtain reasonablypure precipitate products and a nitric acid regeneration process fromnitrates, such as the processes described in U.S. Pat. Nos. 5,912,402and 6,264,909, which are incorporated herein by reference. Whenperforming the processing under atmospheric pressure, a basiccomposition is added to precipitate ferric hydroxide.

In contrast, as described herein, techniques for reclaiming metal valuesfrom metallurgical dust perform the reaction of the metallurgical dustwith nitric acid under pressurized conditions, to form ferric oxide froma metal nitrate solution. A leaching process under pressure produces apurified ferric oxide precipitate with sufficient amounts of othermetals removed such that the resulting material is not toxic waste.Generally, all of the materials throughout the processing arestraightforward to handle.

The improved process described herein can comprise an initial water washof the metallurgical dust, thereby removing some of the chloridecompounds and perhaps other contaminants contained in the dust. Anoptional, preliminary leach of the dust with nitric acid can beperformed under atmospheric conditions, generally prior to performing apressurized leach. The preliminary nitric acid leach is performed at apH at which the iron is insoluble but many of the non-ferrous metals aresomewhat soluble. Removal of the filtrate produces a solid residue at afirst purification level.

The process material for the pressurized leach are solids, which can beunprocessed metallurgical dust or solids obtained following one or moreinitial purification steps, such as washing or acid leaching underatmospheric conditions. The process solid is reacted with nitric acid atenhanced pressure conditions, and generally enhanced temperatureconditions. The acid can be added to the solids under pressurizedconditions or at ambient conditions, although generally the pressure canbe increased following the addition of the acid. Ferric oxide is formedin nitric acid under elevated pressure and temperature conditions. Thus,a significant amount of the iron in the solids can be collected asferric oxide. On the other hand a significant proportion of the othermetals are dissolved by the nitric acid into the solution at theelevated pressure conditions, Since many of the non-iron metals aredissolved, the recovered ferric oxide is significantly purified relativeto the starting metallurgical waste. Thus, the processes describedherein generally produce a ferric oxide solid product with contaminationlevels well below the present EPA toxic waste limits. Any NO_(x) gasthat is generated during this step can be collected and subsequentlyrecycled into nitric acid. The pressurized leach step can be repeatedone or more times to improve the purity of the ferric oxide.Specifically, by using the solid from one pressurized leach in a secondpressurized leach, the remaining non-iron metal is dissolved into thesolution during the subsequent processing step. In some embodiments, theferric oxide that is generated can be sufficiently pure for a variety ofuses including high value applications, such as in a pigment, inmagnetic tape, in a polishing compound and in a variety of other uses.

Since the iron is not dissolved in the pressurized leach step once thepressure and temperature reach their target values, nitric acid is notconsumed as ferric nitrate. Thus, less nitric acid is consumed incomparison with approaches that dissolve the iron in nitric acid andsubsequently precipitate the iron from the solution by adjusting the pH.Generally, an excess of nitric acid is added in the pressurized leachsteps. Thus, the filtrate from each acid leach may contain residualnitric acid. This residual nitric acid can be used by returning thefiltrate, possibly with added nitric acid, to another acid leach step.Once a filtrate has excessive amounts of non-iron metals, the filtratecan be diverted to other processing steps to recover the other metalsrather than to a leach step to use any excess acid. Also, differentportions of the filtrate can be diverted to different uses. e.g., aportion can be recycled for its nitric acid content while anotherportion is diverted for recovery of non-ferrous metals.

The improved process for ferric oxide precipitation involves the use ofa pressure vessel or autoclave that allows for reactions to occur atelevated temperatures and elevated pressures. Suitable pressure vesselsfor operating under appropriate high temperature and high pressure areavailable from various suppliers or can be constructed appropriately.For example, titanium reactors with an appropriate pressure capabilitiescan be used. The size of the pressure vessel can be scaled, for example,to handle the desired quantity of metallurgical dust to treat at a giventime. The pressure vessel can be piped for the transfer of materialsinto and out of the vessel, or materials within other containers can betransferred into and out from the pressure vessel manually. In someembodiments, the vessel can be piped for the transfer of NO_(x) gases toa nitric recycle process. The particular vessel that contacts thesolutions generally have an interior surface that is designed towithstand contact with concentrated acids, such as nitric acid. Interiorsurfaces that may be suitable for such a pressure vessel includetitanium, ceramic, glass and the like. The vessel may or may not providefor agitation of the materials during processing.

In general, the metallurgical dust treatment can be performed in thevicinity of an EAF facility, a BOF facility or other dust generatingfacility, or the dust can be transported to a central recovery facility.If the reclamation process described herein is practiced near anindividual dust producing locations, the transportation of hazardouswaste and the potential liabilities associated with the shipment ofhazardous and noxious wastes can be reduced or eliminated. In addition,the need for storage of such hazardous waste can be reduced andpotentially eliminated.

The improved method of treating metallurgical dust and recovering usedchemicals described herein is based on the differential solubilities ofmetal compounds in a nitric acid solution under elevated temperaturesand pressure. In particular, insoluble ferric oxide forms at highpressures in an aqueous nitric acid/metal nitrate solution. For example,iron nitrate hydrolyzes and precipitates at elevated temperature andpressures. The reaction for the formation of ferric oxide is as follows:2Fe(NO₃)₃+3H₂O→Fe₂O₃+6HNO₃In the present reaction, we do not need to consider whether or not ironnitrate is a formal intermediate in the process, but the end result ofthe processing is that ferric oxide is formed. Thus, the processcomprises reacting the metallurgical dust or a partly purified formthereof with a solution of nitric acid under enhanced temperature andpressure, which can result in the complete or nearly completedissolution of the non-iron metal compositions, such as zinc, manganese,cadmium and lead compositions, that are present in the metallurgicaldust. Filtering of the solid results in the separation of the solidferric oxide from the filtrate, which generally contains dissolvednon-ferrous metals. Other metal values can then be recovered from thefiltrate by further processing, and any unreacted nitric acid in thefiltrate can be reused, if desired.

The reclamation process is summarized in FIG. 1. The reclamation processcan include an optional step of washing 100 the metallurgical dust withwater. The washed dust is filtered 102 to separate the residual solidfrom the wash water. The process also optionally includes a preliminaryleach or pre-leach 104 with nitric acid that is performed at ambientpressure. The pre-leach slurry is filtered 106 with the residue solidsdiverted for additional leaching and the filtrate liquid being divertedfor further processing to recover non-ferrous metals. Additionalpreliminary treatments can also be performed, if desired. The residuefrom the one or more preliminary treatments is subjected to apressurized nitric acid leach 108. The slurry from the nitric acid leachis filtered 110. The solids from the pressurized nitric acid leach arepurified ferric oxide that are used in the purified form or optionallysubjected to an additional pressurized leach to further purify thematerial, as described further below. The filtrates from the variousleach steps can be diverted to reuse residual nitric acid or to furtherprocessing for the recovery of non-ferrous metal components. The furtherprocessing of the filtrates is described further below. Also, theresidue solids can be subjected to one or more additional pressurizedleach steps, as described further below. Each of the steps individuallyor groups of steps collectively can be performed in batch mode oralternatively in continuous operation.

In some embodiments, it can be beneficial to perform this initial stepof washing 100 the metallurgical dust with water prior to the additionof nitric acid. In particular, the water wash can remove undesirablemetal halides and other soluble compositions from the solids such thatthey are not present in later processing steps. In general, sufficientwater can be added to the metallic waste to remove the desiredcompounds. Generally, mixture is stirred to facilitate thesolubilization process. This wash can be performed in any reasonablevessel.

The water wash mixture can be subjected to solid-liquid separation 102to separate a washed metallurgical dust residue from the filtratesolution. Separation process 102 can be performed by pressurefiltration, vacuum filtration, gravity filtration as well as decantingthe liquid from the solid, optionally after centrifugation. Forfiltration approaches, standard commercial filter media can be used,such as polymer woven cloth filter media, paper filter media, porousceramic filter media, and the like. Regardless of the separationapproach, for convenience the separated liquid is referred to as thefiltrate, and the separated solids are referred to as the precipitate.The filtrate then can be sent to a water treatment system forprocessing. The residue solids can be further processed in an acidleach, which can be a pre-leach 104 or directly a pressurized acid leach108.

Either washed or unwashed metallurgical dust can be optionally reacted,i.e., leached, 104 with a nitric acid solution at atmospheric pressure.Sufficient nitric acid generally is added to lower the pH to values inwhich significant quantities of non-ferrous metals are dissolved intothe acid solution without dissolving a significant quantity of the ironcompounds. The pH can be adjusted to be between about 0.25 and about2.5, in other embodiments from about 0.35 to about 2.0, and in furtherembodiments from about 0.5 to about 1.5. The amount of iron that remainsin the solid residue generally is at least about 95 weight percent, infurther embodiments at least about 97 weight percent, and in additionalembodiments at least about 99 weight percent. A person of ordinary skillin the art will recognize that additional ranges of pH and iron contentare contemplated and are within the present disclosure.

In principle, one or more additional preprocessing steps can beperformed to prepare the solid metal waste prior to performing the highpressure acid leach step. Similarly, the optional water leach and/or thepre-leach may or may not be performed. However, the selected solid isthen subjected to a high pressure nitric acid leach step 108. Generally,the materials are also subjected to elevated temperatures during thisstep. During this step, a mixture is formed as a paste or slurry fromthe nitric acid solution and the metal dust waste. The water and/or acidmay or may not be added at the high pressure and temperature conditionsto achieve the desired results. Furthermore, a portion of the waterand/or acid can be added at ambient pressure and an additional portioncan be added at an elevated pressure. The order of performing this stepmay depend on the selected apparatus to perform the processing since itmay be more or less difficult to add materials into the pressurizedcontainer. The mixture can be stirred to facilitate the solubilizationof the non-ferrous metals.

The amount of nitric acid added is a function of the concentration ofthe acid and the volume of acid solution. While in principle pure nitricacid can be used, it can be difficult to handle and expensive to useundiluted nitric acid. In general, the metallurgical dust is reactedwith a nitric acid solution having a concentration in the ranges fromabout a 10 weight percent to about a 75 weight percent nitric acidsolution, in some embodiments, from about 20 weight percent to about 70weight percent, and in further embodiments, from about 25 weight percentto about 65 weigh percent nitric acid solution. The nitric acid can besupplied from a nitric acid supply, which may comprise recycled nitricacid and/or a fresh supply of nitric acid. With respect to relativequantities on compositions, relative weights of nitric acid and metalwaste generally depends on the concentration of the nitric acidsolution. Also, if a more concentrated nitric acid solution is added toform the pulp, additional water may also be added. For more dilutenitric acid solutions, a weight ratio of nitric acid to themetallurgical dust may be, for example, a ratio of about three-to-1(3:1) or greater by weight acid solution to dry metallurgical dust. Inother embodiments with a higher acid concentration, a weight ratio maybe used of one-to-one (1:1) or less of acid solution to drymetallurgical dust. In general, the desired amount of acid added mayalso depend on the composition of the metal dust. The amount of acid canbe evaluated by examining the acid in the resulting mixture. Theconcentration of acid in the resulting mixture can be selected to yieldthe desired solubilization of the non-ferrous metals into the filtratesolution. Acceptable leaching can be accomplished with no free acid inthe mixture. However, generally higher purity ferric oxide is obtainedwhen the mixture contains free nitric acid. The concentration of freenitric acid can be at least 5 grams/liter (g/L), and in some embodimentsat least about 15 g/L, in further embodiments at least about 30 g/L andin additional embodiments at least about 50 g/L. Although the free acidcan be measured at various times in the solubilization process atelevated pressures, these values can be considered the equilibriumvalues after a sufficient period of time that the concentration nolonger changes significantly. A person of ordinary skill in the art willrecognize that additional ranges of reactant acid concentrations, weightratios of acid to solid quantities and free acid concentration withinthe explicit ratios above are contemplated and are within the presentdisclosure.

The amount of acid added can involve trade-offs with respect to cost andresults. The addition of more acid increases the cost for the acid, butthe addition of more acid can result in better solubilization of thenon-iron metals. Better solubilization of the non-iron metals results ina more pure ferric oxide product. In particular, to obtain the morecomplete dissolving of lead and zinc, the presence of free nitric acidmay be desirable during and following the nitric acid leach 108. Iflower amounts of nitric acid are used, undissolved ferrites may remainin the solids. However, multiple nitric acid leaches may be performed todissolve the non-iron metals, whether or not in the form of ferrites,and to recover purer precipitates of ferric oxide.

The high pressure nitric acid leach process can involve one or moresteps within the process. For example, to perform the high pressureleach, the metallurgical dust and the nitric acid can be combined atambient temperature and pressure to form a slurry or pulp. The mixturecan be performed in a pressure vessel, or the mixture can be formed inanother vessel and subsequently transferred to the pressure vessel forperforming the high pressure leach. In some embodiments, themetallurgical dust and nitric acid can be combined at elevatedtemperature and/or pressure, for example the temperature and pressureused to precipitate the ferric oxide.

In a particular embodiment, the nitric acid and metal dust generally ismixed initially at ambient pressures. This mixing may or may not takeplace within the pressure vessel. If the mixing is initially performedoutside of the pressure vessel, the mixture is transported into thepressure vessel. The pressure can be increased by closing the pressurevessel and increasing the temperature. Steam can be injected toeffectuate the increase in both the temperature and the pressure.Alternatively or additionally, the reactor can be heated with a heatingmantle or the like and/or the slurry can be heated during the transferinto the reactor vessel.

Upon forming the combination of dust and acid, the mixture can be mixedat ambient pressure in some embodiments for at least about one quarterhour and in further embodiments from about half an hour to about fivehours and in further embodiments from about one hour to about two hours.Also, the reaction time under elevated pressure should be selected toachieve the desired solubilization of the nonferrous metals into thenitric acid solution. In general, after a sufficient period of time, themixture reaches equilibrium such that the composition does not changesignificantly with the passage of additional time. In the examplesbelow, the composition generally stopped changing after about 2 to about2½ hours. Generally, the Leach within the selected pressure andtemperature ranges is performed for at least about 30 minutes, inadditional embodiments for at least about one hour, in furtherembodiments for at least about two hours, and in other embodiments fromabout 2½ hours to about 5 hours. A person of ordinary skill in the artwill recognize that additional reaction times within the explicit rangesof reaction time are contemplated and are within the present disclosure.Mechanical impellers or other mixing apparatuses can be used to mix theslurry.

In addition, the elevated temperatures and pressures used during thehigh pressure nitric acid leach 108 step increase the dissolution of thezinc, lead and other non-ferrous metals. The pressure and temperaturemay also influence the rates of dissolution. Over appropriate ranges ofthe elevated temperatures and pressures of the nitric acid solution inthe pressure vessel, ferric oxide is almost completely insoluble suchthat almost all of the iron is recovered as ferric oxide. Thus, theferric oxide can be separated from the non-iron metals that remain insolution. Impurities can result, for example, from solid ferrites thatmaintain non-iron metals within the solids. In some embodiments, thehigh pressure leach reaction generally takes place at temperatures inthe range of at least about 150° C., in further embodiments from about200° C. to about 500° C., in other embodiments from about 225° C. toabout 400° C., and in further embodiments from about 250° C. to about350° C. In addition, the pressure of the pressure vessel generally canbe maintained at a value of at least about 225 psig, in furtherembodiments from about 250 psig to about 800 psig, in other embodimentsfrom about 275 to about 600 psig, and in some embodiments from about 300psig to about 500 psig. The psig (pounds per square inch gauge) is ameasure of pressure such that the stated value is the amount aboveatmospheric pressure. For batch processing, the above values may beaverage values potentially following a transient period in which thetemperature and pressure ramp up to near the average processingconditions. A person of ordinary skill in the art will recognize thatadditional ranges of temperature and pressure within the explicit rangesare contemplated and are within the present disclosure.

The high pressure nitric acid leach step 108 can generate gas, NO_(x)which can be vented for transportation to a location for collection andrecovery of nitric by combining the gas with water. The venting of anynitric oxide gases should be performed with due regard for maintainingthe pressure in the pressure vessel at desired levels. Similarly, thecollected nitric oxide gases can be isolated from the pressure vesselfor recovery, for example using conventional pressure valves. The nitricgases represented by NO_(x) can be converted into nitric acid for reuse,for example, following to procedure described in U.S. Pat. No. 6,264,909to Drinkard, Jr., entitled “Nitric Acid Production And Recycle,”incorporated herein by reference.

The residual solid comprising mostly ferric oxide is separated 110 fromthe filtrate. In addition, the residual solid can be washed to removesolubilized non-ferrous metals that stick to the solids during theseparation step. In general, the washing can be performed in anyreasonable approach. In one embodiment, one or more washing steps areused in which each washing step involved suspending the solids in waterand filtering the solids. Enough water can be used to perform thesuspension in each step. The wash steps can be repeated until the washwater has a desired level of purity. The wash water can be added to theinitial filtrate, separately processed to remove the metal components orotherwise treated for disposal.

The filtrate from the high pressure leach generally has free nitric acidas well as significant quantities of solubilized non-ferrous metals.This solution can be alternatively returned to perform additional leachsteps at ambient pressure or at elevated pressures to make use of thefree nitric acid. Additional nitric acid can be added from a nitric acidsupply to obtain the desired levels of acid for the leach. Additionallyor alternatively, a portion or all of the filtrate can be directed tofurther processing to reclaim the non-ferrous metals from the solution,as described further below. FIG. 1 shows the alternative processingpathways for the filtrate solution.

FIG. 2 expands upon the improved reclamation process shown in FIG. 1 byshowing the addition of another pressurized nitric acid leaching step112, in which the ferric oxide solids from the first pressurized nitricacid leaching step are contacted with additional nitric acid andsubjected to elevated pressures, generally with the addition of heat. Inthe additional leaching step 112, nitric acid can be added to the ferricoxide precipitate either under elevated pressure or added at ambientpressure and subsequently reacted under pressure. The additional nitricacid leaching step 112 generally is conducted similarly to the initialnitric acid leaching step 108 described above. Generally, pressurizednitric acid leach is conducted with free nitric acid present. The freenitric acid levels can be comparable to those levels described above forleaching step 108. Since the products have already been purified, loweramounts of acid may be needed for pressurized leach 112. Additionalferrites may be dissolved in this second leach, resulting in a purerferric oxide product. Any NO_(x) gases produced at this stage can berecycled according to the nitric acid recovery.

After sufficient time has passed, the materials from the secondpressurized leach are subjected to filtration 114. The solids can alsobe washed with various amounts of water, as described above with respectto filtration step 110. The solids/precipitate from the filtration arepurified ferric oxide. The filtrate from this second leach may or maynot be combined with the filtrate from an earlier leach step, and theindividual or combined filtrates can be reused, optionally with theaddition of further nitric acid, in another leach step or furtherprocessed for recovery of non-ferrous metals. Portions of the filtratecan be processed differently, as desired. One or more additionalpressurized leach steps can also be performed to further increase thepurity of the ferric oxide solid product.

Using one, two or more pressurized leach steps, the resulting ferricoxide can have many metal contaminant levels reduced to values as low asdesired. With two pressurized processing steps, it is possible to obtainferric oxide without most of the non-ferrous metals. Chromium evidentlyis very difficult to separate from the ferric oxide and co-precipitateswith the ferric oxide. But the other heavy metals generally are removedin the pressurized nitric acid leach to purify the ferric oxide. Pureferric oxide (Fe₂O₃) has 69.94 weight percent iron and 30.06 weightpercent oxygen.

The differential solubilities are sufficient that at least about 95percent, in some embodiments at least about 97.5% and in furtherembodiments at least about 99% of the initial iron in the metallurgicalwaste fed into the pressurized nitric acid leach can be recovered aspurified ferric oxide. In general, it is possible to obtain a solidproduct with at least about 70 weight percent, in further embodiments atleast about 80 weight percent ferric oxide and in other embodiments fromabout 85 to about 95 weight percent ferric oxide. In some purifiedferric oxide materials, a majority of the remaining impurities comprisesilicates. At the same time, it is possible to reduce zinc levels tovery low values, such as levels of no more than about 5% of the initialzinc. Specifically, zinc metal concentrations in the ferric oxide solidscan be reduced to values no more than about 10 weight percent, infurther embodiments no more than about 2 weight percent. In otherembodiments no more than about 0.5 weight percent and in additionalembodiments no more than about 0.1 weight percent. A person of ordinaryskill in the art will recognize that additional ranges of compositionswithin the explicit ranges are contemplated and are within the presentdisclosure.

Similarly, lead, cadmium and manganese can be reduced to lowconcentrations. In particular, toxic lead levels can be reduced to nomore than about 0.1 weight percent, in other embodiments no more thanabout 0.05 weight percent and in further embodiments no more than about0.04 weight percent. Other metal levels, such as arsenic, cadmium,manganese, can be similarly reduced. A person of ordinary skill in theart will recognize that additional ranges of concentrations arecontemplated and are within the present disclosure. Since the heavymetal concentrations can be reduced significantly in the purified ferricoxide solids, there are many options for handling the solid product,including disposing of the solid as regular waste, directing the productback into the steel making operation or using the product in highervalue uses, such as for pigments. For use as pigments, the purifiedferric oxide can be incorporated, for example, into a coatingcomposition by combining the ferric oxide with a suitable carrierliquid. Alternatively or additionally, the purified ferric oxide can becombined with a molding composition for molding into a solid objectincorporating the ferric oxide as a pigment. The molding composition cancomprise, for example, a polymer or concrete. The forming of the solidobject can be based on any of a variety of approaches including, forexample, any of various molding approaches, extrusion approaches, andthe like.

A significant result of the process is that the product dust may nolonger be classified as toxic waste under current standards of theEnvironmental Protection Agency (EPA) under the Toxicity CharacteristicLeach Procedure based Toxicity Characteristic metal waste limits forland fills as found presently in the Code of Federal Regulations. 40C.F.R. § 261.24 (Toxicity Characteristics), incorporated herein byreference. In particular, it has been demonstrated that sufficientamounts of heavy metals can be removed with a single pressurized nitricacid leach, as described herein, that the resulting solids are no longertoxic waste under 2004 EPA solid waste standards, incorporated herein byreference. Specifically, sufficient amounts of arsenic, barium,chromium, mercury, nickel and lead are removed. The ability to treat thedust to form a solid that is not hazardous waste is an important resultof the process. Of course, with appropriate processing, the processesdescribed herein can further improve the purity of the resulting ferricoxide not only to have a material that is not hazardous waste, but thatis suitable for desired uses such as pigments. In particular,crystalline ferric oxide can be produced from the processes describedherein that have desirable characteristics for a variety of uses, suchas pigments, which further benefit from the crystallinity of thepurified ferric oxide. Substantial crystallinity can be verified usingx-ray diffraction analysis of the product materials.

The filtrate following removal of ferric oxide comprises nitric acid anddissolved non-ferrous metal compounds, although some residual ironremains in the solution, which is a non-trivial amount even if only asmall fraction of the original iron in the dust. Generally, asignificant fraction of the remaining metal is zinc. The remainingmetals can be recovered to produce useful materials.

Additional metals can be reclaimed from the metal nitrate filtratesolution. In one pathway, the filtrate, F, from the iron recovery isevaporated 140, generally by applying heat, to concentrate the solution.Generally, most of the non-zinc metals precipitate prior to the zincnitrate. Therefore, the concentrate can be collected and used forfurther processing to recover other metals. Alternatively oradditionally, the solution can be evaporated to dryness and decomposedto produce metal oxides, oxygen and nitrogen oxide gases (NO_(x)) whichcan be recovered and added to water to form nitric acid. The metaloxides can be further processed to recover desired metals. Approachesfor the recovery of nonferrous metals is described further in U.S. Pat.No. 5,912,402 to Drinkard, Jr., et al., entitled “Metallurgical DustRecycle Process,” incorporated herein by reference.

Apparatus

Generally, the various steps of the metal recovery process can beperformed with commercially available reactors and vessels. Thepressurized nitric acid leach steps combine various reactants andequipment to accomplish the ferric oxide purification. An embodiment ofan apparatus for performing the pressurized nitric acid leach fortreating metallurgical dust can comprise metallurgical dust and aqueousnitric acid in a sealed reaction vessel capable of withstanding thepressures reached during the process. The vessels used for at some ofthe other steps of the reclamation process do not necessarily have to bepressure vessels

The metallurgical dust can be supplied from an Electric Arc Furnace(EAF), a Basic Oxygen Furnace (BOF) or some other process that produceswaste metallurgical dust. The water used in the apparatus to wash themetallurgical dust can be city water, deionized water, some otherprocessed water or a combination thereof

Referring to FIG. 3, a schematic diagram shows a reactor suitable forperforming the pressurized nitric acid leach. Reaction system 200comprises a pressure vessel/reactor 202, a paste or pulp 204 of theaqueous nitric acid and metallurgical dust, a heating element 206, andan impeller 208 for stirring the reaction mixture/pulp. Suitablereaction systems can be adapted from commercial reactors or can beconstructed from readily available materials. For example, suitablereactors include, for example, pressure reactors available from ParrInstrument Co., Moline, Ill.

The nitric acid solution can be provided from a nitric acid supply,which can be provided from commercial sources, the nitric acid recyclesystem which recovers nitrogen oxide gases that are hydrated to formnitric acid, filtrate solutions from a leach step that has free nitricacid, or a combination thereof.

Seeded Precipitation Process

In one embodiment of the seeded precipitation process as illustrated inFIG. 7, iron is dissolved in acid to form an iron salt solution in theacid leach stage. Suitable acids include nitric acid, sulfuric acid andhydrochloric acid. Iron can be solubilized and precipitated from ferricsulphate salt or iron chloride solutions. For example, see “Formation ofPure Hematite by Hydrolysis of Iron (III) Salt Solutions underHydrothermal Conditions”, B. Voigt and A. Gobler in, Crystal ResearchTechnology, Vol. 21, 1986, pp.1177-1183). Seed solids are added to thepressure iron precipitation stage, and the mixture is subjected toelevated temperature and pressure for a time sufficient to obtainprecipitates of selected particle sizes.

In another embodiment, iron solids, such as magnetite, are combined withan acid, for example nitric acid, and leached in stirred reactors tosolubilize the iron and form an iron salt solution (400). The exactleach stage arrangement will depend on the characteristics of the ironsolids that are used. The leach may be performed in a single stage or inmultiple stages arranged in “series” or “parallel”. Recycling ofpartially leached solids from the leach stage product stream back to thefeed end of the leach stage may be of benefit to increase ironsolubilizations.

In various embodiments, there is provided the use of magnetite as theiron seed solid in combination with the feed solution for the acidleach. Acid leaching could also be accomplished using otheriron-containing seed solids. These may include scrap irons and othermaterials such as metallurgical wastes or mine concentrates. Forexample, waste solutions from the pickling of steel products. Optionallyiron salt solutions of nitrates, sulphates and chlorides can also beused. Depending on the seed solids utilized, the exact leach circuitarrangement may be modified accordingly.

A certain portion of insoluble material may be contained in the seedsolids. After acid leaching (400), the iron salt solution may besubjected to a liquid/solids separation stage. This can be accomplishedusing a number of conventional techniques including filtration (vacuumand pressure), sedimentation or a combination of both techniques. Anyinsoluble solids can be processed in a high temperature denitrificationstage (401) to remove residual moisture and nitrates. The temperaturesrequired for denitrification will depend on the level of residualnitrates that are acceptable and generally range from about 400° C. upto about 700° C. Moisture and nitrate gases removed during (401) can berecovered for reuse using conventional condensation and scrubbingtechnologies.

The iron salt solution from the leach stage (400) may then be utilizedas the feed solution for the pressure iron precipitation stage (402).Prior to precipitation, iron oxide seed solids may be combined with theiron salt feed solution, in a mixed vessel, to form a feed solution/seedsolid slurry. The quantity of seed solid utilized and/or seeding ratiocan be adjusted in order to control the particle size of theprecipitated iron oxides in the product slurry. The seeding ratio refersto the ratio of the weight of the seed solid to the weight of iron oxideprecipitate that would be expected to be obtained from an unseededpressure precipitation reaction under selected precipitation conditionswith a known feed solution. The seeding ratio is calculated based on theweight of precipitate product from a trial unseeded pressureprecipitation reaction. For example, one would conduct an unseededpressure precipitation reaction using a known volume of feed solutionhaving a known concentration of dissolved iron, at a given temperatureand pressure and weigh the resulting ferric oxide precipitate of thereaction to determine the efficiency of precipitation, i.e. theproportion of dissolved iron that is precipitated under these selectedunseeded conditions. Based on the weight of the product, a selectedseeding ratio can then be selected for subsequent seeded pressureprecipitation reactions. The seeding ratio may, for example range fromabout 20% to about 2000% of the new iron oxides that will beprecipitated. In another embodiment, the seeded precipitation processmay be carried out using a seeding ratio of from about 50% to about500%. Alternatively, all or a portion of the seed material can beinjected directly into the pressure precipitation reactor during theprecipitation process. In another embodiment, the seed solids can beground to pigment grade size of approximately less than or equal to 2microns, prior to being added to the pressure precipitation stage toimprove the control of the precipitated product particle size.

In alternative embodiments, the seeded precipitation process may becarried out using a seeding ratio in the range selected from any minimumvalue of from about 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130,140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260, 270,280, 290, 300, 310, 320, 330, 340, 350, 360, 370, 380, 390, 400, 410,420, 430, 440, 450,460, 470, 480, 490, 500, 510, 520, 530, 540, 550,560, 570, 580, 590, 600, 610, 620, 630, 640, 650, 660, 670, 680, 690,700, 710, 720, 730, 740, 750, 760, 770, 780, 790, 800, 810, 820, 830,840, 850, 860, 870, 880, 890, 900, 910, 920, 930, 940, 950, 960, 970,980, 990, or 1000% to any higher maximum value of about 1000, 1100,1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, or 2000%.

In various embodiments, the pressure iron precipitation stage (402) canbe performed in either a batch-wise or continuous manner. In variousembodiments, a stirred pressure precipitation reactor is utilized tomaintain the seed solids in suspension during the precipitation process.In another embodiment, using a continuously operating precipitationprocess, the reactor may be constructed with internal weirs to dividethe vessel into a number of compartments in series, with eachcompartment agitated individually. Alternatively, any number ofindividually agitated vessels could be arranged in series. One skilledin the art would appreciate that other reactor arrangements are alsocontemplated to achieve the continuous precipitation process.

In various embodiments, the iron salt feed solution/seed solid slurrycan be heated prior to entering the precipitation reactor and/or whilethe precipitations are taking place. Heating can be accomplished eitherdirectly or indirectly. In some embodiments, heat may be recovered fromthe precipitation product slurry (402) and used to heat the nextfeed/seed slurry. The extent of iron precipitation may be dependent onthe temperature utilized during precipitation and the concentration ofiron and free acid in the feed slurry. As the concentration of dissolvediron and free acid are increased, the temperature required to obtain thesame level of precipitation is also increased. In one embodiment,conventional process vessel design economics typically dictatetemperatures of from about 100° C. to about 300° C. with the feed/seedslurry containing anywhere from about 5 g/l of dissolved iron up to theonset of crystalization of the ferric salt. A similar amount of freeacid could also be used in some embodiments. These conditions wouldrequire pressures of from above atmospheric pressure to about 1300 psig.Other reactor vessel designs may allow for higher concentrations of ironand free acid and correspondingly higher temperatures to be utilized. Ingeneral, the seeded process is a rapid reaction. The precipitationreactions proceed to completion quite rapidly, in some cases requiringretention times of less than 1 hour. Higher precipitation temperaturesmay reduce the reaction time in some embodiments. In variousembodiments, the seeded precipitation process may have retention timesfrom about one minute to about six hours. In various embodiments, theseeded precipitation process may have retention times from about thirtyminutes to about one hour.

In alternative embodiments, the seeded precipitation process may haveretention times selected from any minimum value of about 1, 2, 3, 4, 5,6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24,25, 26, 27, 28, 29, or 30 minutes to any higher maximum time of about30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 110, 120,130, 140, 150, 160, 170, 180, 190, 200, 210, 220, 230, 240, 250, 260,270, 280, 290, 300, 310, 320, 330, 340, 350, or 360 minutes.

In various embodiments, the seeded precipitation process may utilizetemperatures from about 100° C. to about 300° C. In other embodiments,temperatures of from about 175° C. to about 250° C. may be utilized. Inother embodiments, lower temperatures of from about 130° C. to about175° C. can be used, especially if solutions having a lower ironconcentrations are used. In alternative embodiments, the seededprecipitation process may be carried out at a temperature, ortemperature range selected from any minimum value of about 100, 105,110, 115, 120, 125, 130, 135, 140, 145, 150, 155, 160, 165, 170, or 175°C., to any higher maximum value of about 175, 180, 185, 190, 195, 200,205, 210, 215, 220, 225, 230, 235, 240,245, 250, 255, 260, 265, 270,275, 280, 285, 290, 295, or 300° C. In various embodiments, thepressures associated with temperature ranges of from 175° C. to 250° C.are from about 100 psig to about 600 psig.

In alternative embodiments, the seeded precipitation process may becarried out at a pressure, or pressure range selected from any minimumvalue of about 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70,75, 80, 85, 90, 95, 100, 105, 110, 115, 120, 125, 130, 135, 140, 145,150, 155, 160, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215,220, 225, 230, 235, 240, 245, 250, 255, 260, 265, 270, 275, 280, 285,290, 295, or 300 psig, to any higher maximum value of about 300, 305,310, 315, 320, 325, 330, 335, 340, 345, 350, 355, 360, 365, 370, 375,380, 385, 390, 395, 400, 405, 410, 415, 420, 425, 430, 435, 440, 445,450, 455, 460, 465, 470, 475, 480, 485, 490, 495, 500, 510, 520, 530,540, 550, 560, 570, 580, 590, 600, 610, 620, 630, 640, 650, 660, 670,680, 690, 700, 710, 720, 730, 740, 750, 760, 770, 780, 790, 800, 810,820, 830, 840, 850, 860, 870, 880, 890, 900, 910, 920, 930, 940, 950,960, 970, 980, 990, 1000, 1010, 1020, 1030, 1040, 1050, 1060, 1070,1080, 1090, 1100, 1120, 1130, 1140, 1150, 1160, 1170, 1180, 1190, 1200,1210, 1220, 1230, 1240, 1250, 1260, 1270, 1280, 1290, or 1300 psig.

In other embodiments, temperatures can be modulated to control particlesize and appearance. For example, higher temperatures generally resultin larger particles with smoother surface textures while lowertemperatures may produce particles with rougher surfaces. In someembodiments, lower temperatures at a given seeding ratio may result insmaller particles than a correspondingly higher temperature.

In various embodiments, after the pressure precipitation stage (402),the product slurry may be discharged from the reactor where it may becooled and returned to atmospheric pressure. Cooling can occur viaindirect means, direct injection or steam flashing as the pressure isrelieved. The product slurry stream may then be subjected to asolid/liquid separation process. This can be accomplished using a numberof convention techniques including filtration (vacuum and pressure),sedimentation or a combination of both techniques. In anotherembodiment, a portion of the precipitated solids may be diverted andrecycled as seed solids for the pressure precipitation. In anotherembodiment, the final product solids may be denitrified/calcined (403)to remove any residual water and nitrates. The temperatures required fordenitrification will depend on the level of residual nitrates that areacceptable. Temperatures for denitrification/calcination may generallybe in excess of from about 400° C. up to about 700° C. Moisture andnitrate gases removed during this stage (403) may be recovered for reuseusing conventional condensation and scrubbing technologies. In anotherembodiment, the dried solids may then be processed conventionally aspigment material.

During the pressure precipitation process, the iron in solution may behydrolyzed to form ferric oxide solids. In one embodiment, the acidcomponent of the species may be recovered or regenerated in solution,for example, in the form of nitric acid. Following liquid/solidseparation, the majority of this solution may be recycled to the acidleach stage (400) in order to produce new ferric nitrate solution forfurther pigment production. In some embodiments, the bleed stream may beutilized to remove a portion of the regenerated acid solution. Thepurpose of this bleed to maintain acceptable levels of other non-ferrousminor impurities that may be solubilized in the acid leach stage (400).The ratio of bleed volume to recycled solution volume may be dependenton the quantities of these impurities in solution. Impurities depend onthe source of iron feed material being utilized and may typicallyinclude elements such as manganese, aluminum, calcium, magnesium,sodium, etc.

In various embodiments there are several potential treatment processesavailable for the nitrate solution bleed stream. In one embodiment, thestream may be subjected to a distillation process (404) to concentratethe acid and soluble metal nitrates by removing an overhead stream ofwater/weak nitrates. The distillation bottoms may then be subjected to adirect spray drying process (405) which flashes off the majority of thenitric acid and water leaving a solid product of metal oxides with someresidual nitrates and moisture. The spray dryer overheads may be treatedvia condensation and scrubbers to recover the nitric acid which may thenbe recycled to the acid leach stage (400) of the process. In anotherembodiment, the solids from the spray drying process may be furthertreated to remove the residual nitrates/moisture by denitrification(401).

Alternatively in another embodiment, the nitrate bleed stream may betreated via alkaline (i.e. lime) precipitation to precipitate themajority of the metallic impurities (406). The slurry from theprecipitation may be processed via solid/liquid separation to remove theprecipitated solids which may then be subjected to a denitrificationstage (401) to eliminate residual nitrates and moisture. The solution(filtrate) from the solid/liquid separation may be concentrated viaevaporation (407) to produce a commercial grade calcium nitrate solutionthat can be sold. Depending on the choice of alkaline for theprecipitation, different final product solutions can be prepared. Thechoice of processing solution for the nitrate bleed solution will dependon a number of project-specific parameters. The goal of the treatmentsis to recover value for the contained nitrates and produce products thathave economic value.

In various embodiments, both the seeding ratio and temperature may bemodified to tailor the characteristics of the particle obtained byprecipitation. Modifications of the seeding ratio and temperatureresults in the production of a wide range of pigment colours and sizes.For example, red shades from “yellow-shade” reds (fine particles) to“blue-shade” reds (coarser particles) can be obtained. The seededprecipitation process allows control of the size and colour of theprecipitates directly when the particles are being formed. In anotherembodiment, if additional modifications are required, the precipitatescan be processed conventionally by subsequent calcination and/ormilling.

In various embodiments, the precipitates obtained by the seededprecipitation process have average particle size diameters of from about0.1 microns up to about 10 microns. In various embodiments, theprecipitates obtained by the seeded precipitation process have particlesize diameters of from about 0.15 microns up to about 2.5 microns. Inalternative embodiments, the average particle size diameters (d₅₀) ofthe particles of ferric oxide precipitates obtained by the seededprecipitation process are selected from any minimum value of about 0.1,0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5,1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, or 2.5 microns, to anyhigher maximum value of about 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.1, 3.2,3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0,7.5, 8.0, 8.5, 9.0, 9.5, or 10 microns.

In various embodiments, to obtain a salt solution of dissolved iron,magnetite may be dissolved in nitric acid. In another embodiment, ironsolids such as hematite or waste pickling steel products can be usedwith the claimed process. In another embodiment, salts other than ferricnitrates can be used, for example sulphates and chlorides can also beemployed.

In another embodiment, the dissolved iron concentration in the saltsolution can range from about 5 g/L up to the onset of crystallizationof the ferric salt. In another embodiment, the dissolved ironconcentration in the salt solution can range from about 10 g/L to about100 g/L. In another embodiment, the dissolved iron concentration in thesalt solution can range from about 30 g/L to about 60 g/L. Inalternative embodiments, the dissolved iron concentration is selectedfrom any minimum value of about 5, 10 15, 20, or 30 g/L, to any maximumvalue of about 30, 40, 50, 60, 70, 80, 90, 100 g/L or up to the onset ofcrystallization.

In one embodiment, the concentration of free acid in the iron saltsolution can range from about 0 to about 150 g/L. In one embodiment, theconcentration of free acid in the iron salt solution can range fromabout 30 to about 70 g/L. One skilled in the art will appreciate thathigher free acid concentrations can be used but would requirespecialized reactor vessels able to tolerate higher pressures. Inalternative embodiments, the free acid concentration is selected fromany minimum value of about 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55,60, 65, 70, or 75 g/L, to any maximum value of about 75, 80, 85, 90, 95,100, 105, 110, 115, 120, 125, 130, 135, 140, 145, or 150 g/L.

Suitable seed material can be any material the permits the recovery ofprecipitates having the desired characteristics. In one embodiment,commercially available Bayferrox™ 105M or 130M (Lanxess) iron oxidepigments can be used. In various embodiments, seed material can berecycled within the process and externally purchased material will thennot be required. This seed material will be diverted from the productstream from the pressure precipitation stage (402) and will be recycledas required. The recycled seed may be subjected to grinding in order tofurther improve the desired characteristics of the precipitated product.In general, seed ratio refers to the quantity of seed solids versus thequantity of new iron oxide precipitates. In various embodiments, thisprocess utilizes seed ratios from about 20% to about 2000%.

This size range is characteristic of pigment grade iron oxides. Invarious embodiments of the process, as seeding ratio and temperature areadjusted, the particle size of the precipitate can be modulated. As theparticle size increases, the colour shade of the precipitates graduallychanged from a fine “yellow-shade” red to a coarse “blue-shade” red. Aselected particle size is any that is desirable. In some embodiments theselected particle size may be finer and a high seeding ratio, oralternatively a lower temperature may be used in the process. If coarserparticle sizes are desired, a lower seeding ratio, or alternatively ahigher temperature may be used in the process.

In various embodiments, the ferric oxide precipitates obtained from theseeded precipitation process have an L* of about 40 to about 60. Invarious embodiments, the ferric oxide precipitates obtained from theseeded precipitation process have an L* of about 49 to about 55. Inalternative embodiments the ferric oxide precipitates can have an L*selected from any minimum value of about 40, 41, 42, 43, 44, 45, 46, 47,48, 49 or 50, to any maximum value of about 50, 51, 52, 53, 54, 55, 56,57, 58, 59 or 60. In various embodiments, the ferric oxide precipitatesobtained from the seeded precipitation process have an a* of about 10 toabout 40. In various embodiments, the ferric oxide precipitates obtainedfrom the seeded precipitation process have an a* of about 19 to about33. In alternative embodiments the ferric oxide precipitates can have ana* selected from any minimum value of about 10, 11, 12, 13, 14, 15, 16,17, 18, 19, 20, 21, 22, 23, 24 or 25 to any maximum value of about 25,26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39 or 40. In variousembodiments, the ferric oxide precipitates obtained from the seededprecipitation process have a b* of about 5 to about 35. In variousembodiments, the ferric oxide precipitates obtained from the seededprecipitation process have a b* of about 12 to about 28. In alternativeembodiments, the ferric oxide precipitates can have a b* selected fromany minimum value of about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16,17, 18, 19 or 20 to any maximum value of about 20, 21, 22, 23, 24, 25,26, 27, 28, 29, 30, 31, 32, 33, 34 or 35.

EXAMPLES

The metallurgical dust used in the following tests was produced as theresult of a steel making operation and was EAF dust. The EAF dust thatwas provided for the various tests,

Example I Metallurgical Dust Pressure Nitric Acid Leach Tests

These examples demonstrate the feasibility of precipitating ferric oxideand solubilizing other metals from a slurry of metallurgical dust andnitric acid. The processing is performed in a pressurized vessel withthe application of heat with conditions selected to obtain goodseparation of ferric oxide solids. The leach temperatures and free acidlevels in the tests were varied in order to explore the results underdifferent conditions. These tests utilized EAF dust samples with thefollowing components, as determined by an elemental analysis.

TABLE 1 Component Wt. Percent (%) Iron 26.0 Zinc 27.0 Lead 1.4 Cadmium0.081 Manganese 3.4 Chrome 0.25

A quantity (240 g.) of EAF dust was pulped with 400 mL of deionizedwater and mixed with a mechanical mixer in a reactor vessel. The reactorvessel was a Parr pressure reactor (Parr Instrument Co., Moline, Ill.)with stirring. While mixing, 69% nitric acid was slowly added (over aperiod of 30 minutes) to the mixture. The acid was added to produce anoverall acid addition ratio of 1175 g of 69% acid per kg of dry dust.The slurry was allowed to mix for 60 minutes from the time of the lastacid addition. At the end of the mix time, a 25 mL pulp sample wasremoved from the vessel and filtered. The sample solution (filtrate) wascollected and submitted for analysis. The sample filter cake residue waswashed with deionized water, dried, and weighed. A portion of the filtercake was analyzed for composition.

An additional amount of 69% nitric acid was added to the pre-mixed dustslurry prior to introducing the slurry into the reactor vessel. Thematerial in the reactor vessel was mixed for an additional 30 minutes,and then the autoclave vessel was sealed.

The material in the autoclave vessel was heated to 180-220° C. for 3hours, via external vessel heating. The pressure in the vessel wasmonitored such that the vessel was vented if the pressure approached 500psig. Kinetic samples, i.e., intermediate samples, of the autoclavematerial were taken at 1 hour and at 2 hours from the time the materialreached temperature. The kinetic pulp samples were filtered, and thepressure leach solution samples (filtrate) were collected and analyzed.The filter cake residue samples were washed with deionized water, dried,weighed, and analyzed.

At the end of 3 hours, the autoclave was cooled and the pressure wasrelieved. The filtrate was collected and analyzed. The filter cakeresidue was displacement washed 3 times with fresh water. The washsolutions were combined and analyzed. The washed filter cake residue wasdried, weighed and analyzed.

The initial leach (at atmospheric pressure) and the pressure acid leachconditions are summarized below:

Added To Initial Mix Initial Mix For Pressure Acid Leach Feed weight (g)240 Deionized water (g) 200 Deionized water (g) 400 69% nitric acid (g)varied 69% nitric acid (g) 282 Pulp density (% solids w/w) 15-20 Time(min.) 60 Temperature (° C.) 180-220 Pulp Density 26.0 Time (attemperature) (min.) 180 (% solids w/w)

Table 2 provides the key operating conditions for the autoclave leachtests. By the time that the reactor was up to temperature, the pressurein the reactor generally ranged between 360 psig and 500 psig when thereactor was at about 220° C. and greater than 100 psig when the leachingwas performed at 180° C. Table 3 provides results of analysis of thesamples taken during and at the completion of the leaching process.

TABLE 2 PREMIX PAL 69% 69% Nitric Pulp Nitric Pulp Leach (kg/t DensityLeach (kg/t Density % Temp Oxygen Time Test No. feed) % solids Time (h)feed) solids ° C. over psi (h) PAL 1 1175 26 1 467 19.4 220 0 3 PAL 21175 26 1 467 19.4 180 0 3 PAL 3 1175 26 1 700 18.6 220 0 3 PAL 4 117526 1 933 17.8 220 0 3 PAL 5 1175 26 1 933 17.8 220 50 3 PAL 6 1175 26 11050 17.5 220 0 3 PAL 7 1175 26 1 1050 17.5 220 0 3 Note: PAL 7 is arepeat of PAL 5 with a finer grind

TABLE 3 Test Results Retention Free Test Time Filtrate Assays (g/L) AcidAssays (%) No (h) Cd Cr Fe Mn Pb Zn (g/L) Cd Cr Fe PAL 1 Premix  40.0130 0.37 45 1 220 8.7 440 7600 3500 57700 17 0.0030 0.50 58 2 220 1.686 8300 3500 61000 13 0.0021 0.40 50 3 240 3.3 230 8500 3600 63100 130.0023 0.6 58 PAL 4 1 220 46 1100 8400 3800 69000 44 0.0016 0.41 45 2210 6.9 260 8500 3500 68000 36 0.0005 0.48 56 3 220 6.4 500 9100 370072000 40 0.0005 0.58 57 PAL 3 1 200 47 1900 7900 3300 64000 62 0.00220.44 50 2 210 24 700 8000 3300 62000 62 0.0020 0.47 53 3 220 33 18008600 3500 67000 63 0.0025 0.56 56 PAL 6 1 217 70 2690 8770 3470 71000100  0.0001 0.35 53 2 228 44 1540 9420 3660 75300 105  0.0001 0.45 55 3210 85 3930 8460 3310 67900 91 0.0001 0.44 54 PAL 5 2 220 91 1560 90203500 73700 80 0.002 0.37 52 2.5 221 57 640 8950 3500 72500 77 0.002 0.3754 3 221 72 1840 8760 3460 71600 77 0.002 0.49 48 PAL 2 Premix 330 21010000 2900 5700 84000 . . . 0.0120 0.37 41 1 200 250 4700 4200 350055000 55 0.0067 0.28 43 2 220 290 2900 5900 4000 64000 43 0.0054 0.29 473 230 290 2400 8000 3900 69000 23 0.0046 0.32 51 PAL 7 Premix . . . . .. . . . . . . . . . . . . . . . 0.013 0.36 41 1 200 87 1700 8000 310064000 75 0.0018 0.47 54 2 190 76 1200 7700 3000 61000 70 0.0005 0.5 58 3170 60 880 7400 3000 64000 80 0.0005 0.55 59 Retention Test Time Assays(%) Dissolution (%) No (h) Mn Pb Zn Cd Cr Fe Mn Pb Zn PAL 1 Premix 5.600.1 14.0 1 0.99 0.0 3.7 98.5 1.5 0.7 87.3 98.6 93.3 2 0.54 0.0 2.5 99.00.4 0.2 93.3 98.9 95.7 3 0.61 0.0 2.6 98.9 0.5 0.3 92.3 98.7 95.4 PAL 41 0.26 0.036 0.83 99.1 8.0 1.9 96.1 98.8 98.5 2 0.10 0.027 0.16 99.8 1.40.4 98.8 99.2 99.8 3 0.06 0.032 0.20 99.7 1.1 0.8 99.3 99.0 99.7 PAL 3 10.23 0.05 0.98 98.8 8.6 3.2 96.8 98.4 98.3 2 0.12 0.05 0.70 99.0 4.6 1.298.4 98.4 98.8 3 0.14 0.06 0.84 98.8 5.5 3.0 98.3 98.3 98.7 PAL 6 1 0.140.029 0.42 99.5 15.9 4.6 98.3 99.1 99.4 2 0.08 0.013 0.06 99.5 7.8 2.499.0 99.1 99.9 3 0.09 0.020 0.11 99.5 16.9 15.2 99.0 99.1 99.8 PAL 5 20.11 0.030 0.43 99.2 21.3 3.2 98.9 98.4 99.5 2.5 0.03 0.013 0.06 99.217.3 3.5 99.7 98.4 99.9 3 0.11 0.050 0.67 99.9 12.2 3.5 98.7 98.2 99.0PAL 2 Premix 5.10 0.085 13.0 93.7 23.3 11.6 23.4 97.3 77.6 1 3.60 0.05311.0 96.1 42.2 8.2 48.8 98.2 80.3 2 2.90 0.040 9.4 96.8 42.6 4.4 60.298.7 83.5 3 1.40 0.045 5.9 97.5 41.1 3.6 81.3 98.5 90.0 PAL 7 Premix 5.20.078 14 93.1 24.0 11.7 20.2 97.0 74.7 1 0.077 0.024 0.22 99.2 16.8 3.399.1 99.3 99.7 2 0.023 0.021 0.034 99.8 15.0 2.3 99.7 99.4 100.0 3 0.0240.021 0.04 99.8 11.7 1.8 99.7 99.4 100.0Summary of Results for PAL1-PAL7

It appears that, generally, the increased temperature and increased freeacid levels result in more complete dissolution of the EAF dust ferritecontent and, consequently, lower residue solids impurity levels.

Table 4 summarizes the TCLP (Toxicity Characteristic Leaching Procedure)results for Samples PAL1 and PAL 4, and also contains the EPA limits fordelisting a material produced from a hazardous waste.

TABLE 4 As Ba Cr Ni (mg/L) (mg/L) (mg/L) Hg (mg/L) (mg/L) Pb (mg/L) EPAlimit 0.5 7.6 0.33 0.001 1.0 0.15 (delisting) PAL I <0.05 0.10 0.10<0.001 <0.02 0.02 PAL 4 <0.05 0.17 <0.02 <0.001 0.04 0.10

These results demonstrate that the attraction of the non-ferrous metalsexcept for chromium was very high, and generally reached levels of 99+.The final iron product generally included the chrome content of theinitial dust. However, the use of higher acid levels generally resultedin greater removal of the non-ferrous metals, and a corresponding lowerimpurity level. An iron content approaching 60% indicates that the ironwas present in the form of crystalline hematite with a quantity (10-15weight %) dust insolubles (silicates). The recovery of iron was greaterthan 95 weight percent. The levels of zinc, manganese and lead decreasedwith increasing amounts of free acid. The extraction of the non-ferrousmetals into the solution generally seemed complete after about 2 to 2.5hours.

Example 2 Pressurized Leach Results with Second Experimental Setup

These examples demonstrate the feasibility of precipitating ferric oxideand solubilizing other metals from a slurry of metallurgical dust andnitric acid. The processing is performed in a pressurized vessel withthe application of heat with conditions selected to obtain goodseparation of ferric oxide solids. The leach temperatures and free acidlevels in the tests were varied in order explore the results underdifferent conditions. These tests were performed similarly to those inexample 1 using an alternative test setup. These tests utilized EAF dustsamples with the following components, as determined by an elementalanalysis.

TABLE 5 Component Wt. Percent (%) Iron 22.0 Zinc 23.7 Lead 2.1 Cadmium0.073 Manganese 2.87 Chrome 0.20

A quantity of EAF dust (310 g wet weight/249 g dry weight) was pulpedusing 30 minutes of mixing with 748 g of 44.2 weight percent nitric acidto give a slurry weight of 997 g with 25 weight percent solids. Thisinitial mixing was performed prior to heating. These tests were alsoperformed in a pressurized reactor at a temperature of 220 degrees C.for 120 minutes. The maximum pressure for these experiments was 620 psigwith no bleed. As shown in Table 6, this test again demonstrates theability of the pressurized leach process to obtain ferric oxide puritiesbelow the EPA limits such that the materials are no longer hazardouswaste.

TABLE 6 As Ba Cr Hg (mg/L) (mg/L) (mg/L) (mg/L) Ni (mg/L) Pb (mg/L) EPAlimit 0.5 7.6 0.33 0.001 1.0 0.15 (delisting) 2868-12 <0.03 1.6 0.01<0.0001 0.006 <0.073

Example 3 Secondary Leach of Test Residue

Products from a first pressurized leach were subjected to a secondpressurized leach to demonstrate further reductions in the contaminantlevels. One of these tests was performed with products from Example 1and a second test was performed with products from Example 2.

A 35 g dry weight sample of filter cake residue produced from a singleleach process (residue is from PAL6 of Example 1), was repulped with 706g of deionized water and 40 g of 59 weight percent fresh nitric acidsolution and re-leached. The nitric acid was added to the residue priorto the introduction of the mixture to the reactor. The leach slurry wasapproximately 20% solids. A stirred Parr pressure reactor was utilizedwith a steam pressure of about 317 psig. The “second stage” pressureleach tests were performed in a manner similar to the first stage leachtests. Generally, the second leaching step was able to further reducethe impurity levels in the iron residue product of a single-stage leach.Table 7 provides the results from the secondary leach.

TABLE 7 Deg Free Fe Zn Pb Cd Mn Cr Test C. Acid (%) (%) (%) (%) (%) (%)1^(st) Leach 220 91 54 0.11 0.021 0.001 0.088 0.44 (PAL 6) 2nd Leach 22037 59 0.03 <0.02 <0.0005 0.067 0.53 (PAL 6)

A second two stage leach test was also performed based on the productfrom the pressurized leach process as described in Example 2 with theamount of nitric acid as described below. Initial residue was preparedby adding 300 g of water washed dust filter cake (240 g solids dryweight) to 720 g of 27% nitric acid. The slurry was allowed to mix for20 minutes before being added to a Parr pressure reactor. The reactortemperature was then raised to 220 degrees C. and held for 3 hours attemperature. The pressure leach stage resulted in a low final residualfree nitric acid level of 15 g/L which resulted in incomplete dustdissolution. In the second pressurized leach test, 80 g of dry residuefrom this first test was combined with 320 g of 20% nitric acid andreacted in an autoclave for 1.5 hours at 220° C. (2868-8). The finalresidue had low residual impurity levels, as presented in Table 8.

TABLE 8 Free Deg Acid Fe Zn Pb Cd Mn Cr Test C. (g/L) (%) (%) (%) (%)(%) (%) 1^(st) 220 15 42 9.9  0.102 0.009 3.7  0.36 Leach 2nd 220 82 560.10 0.008 N/A 0.08 N/A Leach

The second leach is able to significantly increase the purity of theferric oxide with respect to non-ferrous metals except for chromium.This is true even if the first leach was performed under conditions oflower free acid levels (incomplete dissolution).

Example 4 Preleach Tests

This experiment demonstrates that the performance of a first leach stepat atmospheric pressures can effectively reduce impurity levels prior toperformance of further processing.

The pH of the slurry can be controlled to adjust the purity of theproduct from this atmospheric leach. The test were performed with aslurry with about 30 weight % solids. A 240 g quantity of solids wascombined with 400 g water. Then, 69 weight percent nitric acid was addedto obtain the desired test pH. The test conditions for four runs arepresented in Table 9.

TABLE 9 Pulp Density Leach Temp 69% Nitric Test No. pH % solids Time(min) (° C.) (kg/t feed) NL-3 1.5-2.0 ~30 240 25 572 NL-2 1.0-1.5 ~30240 25 664 NL-4 0.5-1.0 ~30 240 25 725 NL-5 1.0-1.5 ~30 240 80 704 Adenotes feed added to acid solution.

The solid residue from the test was further analyzed. The results of theanalysis are presented in Table 10.

TABLE 10 Retention Test Time Solution Assays (mg/L) Residue Assays (%)Dissolution (%) No (h) Cd Cr Fe Mn Pb Zn Cd Cr Fe Mn Pb Zn Cd Cr Fe MnPb Zn NL-3 0 331 0.735 1.4 940 3760 69000 0.0253 0.34 35 4.7 0.806 1981.1 0.1 0.0 6.2 60.5 54.4 1 330 1.29 11 1100 3910 71000 0.0235 0.36 364.8 0.742 18 83.9 0.1 0.0 7.8 66.2 59.4 2 339 3.5 37 1200 4300 750000.0180 0.38 37 5.1 0.496 17 88.3 0.4 0.0 8.7 77.7 64.0 3 357 0.417 1.51200 4160 73000 0.0181 0.37 37 4.9 0.637 16 87.7 0.0 0.0 8.1 70.2 62.2 4332 0.55 2.9 1200 4070 77000 0.0201 0.40 40 5.2 0.637 18 85.6 0.1 0.07.7 69.8 60.7 NL-2 0 320 12 310 1100 4400 72000 0.0200 0.38 38 4.9 0.6116 86.0 1.2 0.3 8.0 73.5 63.4 1 330 15 420 1300 4900 79000 0.0150 0.4040 5.1 0.48 15 90.7 1.6 0.5 10.1 81.8 69.9 2 310 28 1100 1300 4300 660000.0130 0.38 39 5.2 0.30 15 91.6 3.3 1.3 10.3 86.8 66.9 3 310 32 10001300 4800 75000 0.0140 0.39 40 5.5 0.28 15 92.5 4.4 1.4 11.7 90.6 73.7 4330 22 460 1400 4700 76000 0.0160 0.42 41 5.1 0.40 15 91.5 2.7 0.6 12.686.0 72.7 NL-4 0 320 39 1500 1300 4800 74000 0.0199 0.39 39 5.1 0.43 1686.3 3.7 1.5 9.1 81.5 64.4 1 290 61 3700 1300 4800 72000 0.0144 0.40 405.5 0.29 16 90.7 7.0 4.3 10.3 88.8 68.6 2 340 81 4500 1600 5200 810000.0132 0.39 41 5.5 0.23 15 92.5 9.1 5.0 12.2 91.7 72.1 3 330 120 64001700 5300 82000 0.0130 0.39 41 5.5 0.20 15 92.5 12.9 7.1 13.1 92.7 72.74 320 130 6900 1800 4900 78000 0.0128 0.34 43 5.9 0.18 16 92.2 15.3 7.112.6 93.0 69.8 NL-5 0 277 2.19 21 1100 3680 58000 0.0160 0.38 35 4.90.43 15 89.9 0.3 0.0 10.3 81.5 66.5 1 304 1.23 33 1300 3770 63000 0.01400.38 36 5.0 0.44 15 90.8 0.1 0.0 10.5 79.5 65.5 2 301 116 620 2000 500074000 0.0140 0.40 38 5.2 0.12 13 92.2 13.8 8.3 17.5 95.8 75.9 3 324 14.1230 1800 4930 78000 0.0130 0.42 37 4.8 0.25 13 92.0 1.5 0.3 14.8 90.173.5 4 344 29.2 200 2200 5430 86000 0.0110 0.40 38 4.7 0.17 12 92.9 2.90.2 16.3 93.0 74.9 These results show that the pH can be adjusted todissolve significant quantities of the non-ferrous metals while leavingmost of the iron in the solids. Thus, a more purified solid can be usedin the resulting pressurized leach.

Example 5 XRF (Crystallinity) Data

This example presents x-ray data showing that the recovered Fe₂O₃ is incrystalline hematite form.

Referring to FIGS. 4 and 5, the x-ray diffraction data is shown forSamples Pal 6 and 2868-12 from the examples above. The x-ray diffractiondata was obtained on a Siemens D5000 diffractometer using Co radiation.The major crystalline components from these diffractograms is hematiteFe₂O₃. For comparison, an x-ray diffractogram is shown in FIG. 6 for thematerial prior to performing the leach. This material demonstrates amajority phase of zincite (ZnO) and minor phases of gypsum (CaSO₄.2H₂O),magnetite (Fe₃O₄), pyrite (FeS₂) and pyrrhotite (Fe_((1−x))S) as well astraces of other crystalline forms.

The following examples are related to the modified process.

Example 6 Batch Tests for Seeded Process

The salt solution for the tests was prepared by mixing reagent gradeferric nitrate salts with deionized water to obtain the desired initialdissolved iron concentration. In another embodiment, iron solids aresolubilized in nitric or sulphuric acid. A 69% solution of nitric acidwas added to produce a residual free acid level that would be indicativeof a solution produced by a reaction between nitric acid and an ironsolid such as magnetite. The following series of tests was performedutilizing various reaction conditions. Some experimental parameters thatwere examined include: iron concentration, free acid concentration, seedratio (ratio of iron in seed solids to dissolved iron in initial saltsolution), temperature and reaction time.

A 2 liter Parr heated pressure reactor vessel was used for all tests.The vessel was equipped with a mechanical mixer for agitation. For eachtest, approximately 1000 mL of feed solution was added to the reactoralong with the desired quantity of seed solids. Bayferrox™ 105M(Lanxess) iron oxide pigment material was used as the seed for thetests. The reactor is sealed and heated to a target temperature with anassociated pressure. For example, at 50 g/L of initial iron in solutiontemperatures and pressures of approximately 155 psig at 175° C. up toapproximately 500 psig at 240° C. can be used. Following the completionof the reaction time, the reactor was cooled and the pressure relieved.The slurry was filtered and the filtrate collected and analyzed. Manyconventional solid/liquid separation techniques can be used at thisstage, for example, sedimentation, filtration, centrifugation, etc. Thefilter cake residue was displacement washed with fresh water and thewash solutions were combined and analyzed. The washed filter cakeresidue was dried, weighed and analyzed.

Several tests were conducted to analyze the precipitated solids, such asa metal determination by ICP and size analysis using a MicromereticsSediGraph™ 5100 analyzer. Particle diameters are indicated as d₅₀ whichis the size that 50% of the solids are finer than. The results of theseanalyses are summarized in Table 11. In addition, the solids areanalyzed for colour properties using a CIELAB system. The colour data,summarized in Table 12 and also shown in FIG. 8, was obtained using aDatacolor Spectragraph™ SF450.

TABLE 11 Shown are the various operating conditions used for seededprecipitation tests. Seed Initial Iron Temp Ratio Concentration Initialfree Reaction Time Test (° C.) (%) (g/L) acid (g/L) (hr) PPT 22 200 5049.8 32 1 PPT 27 240 100 49.8 32 1 PPT 23 200 100 49.8 32 1 PPT 1 175200 49.8 32 1 PPT 7 175 200 49.8 32 1 PPT 8 175 300 49.8 32 1 PPT 18 175400 49.8 32 0.5 PPT 20 225 300 49.8 32 1

TABLE 12 A summary of the colour analysis compared to size of theprecipitates obtained from the various test conditions summarized intable 11. Test L* a* b* d₅₀ PPT 22 52.69 25 16.91 1.34 PPT 27 50.4528.01 20.7 0.63 PPT 23 52.28 27.7 21.23 0.46 PPT 1 52.48 27.83 22.230.44 PPT 7 52.68 28.4 23.04 0.46 PPT 8 52.47 28.88 24.23 0.38 PPT 1851.7 29.13 24.98 0.37 PPT 20 51.25 29.3 25.02 0.39 Bayferrox ™ 105M53.15 29.05 24.01 0.60 Bayferrox ™ 130M 50.67 25.97 16.46 0.70

To measure the L*a*b* parameters of the test precipitates, U.S.Stoneware jar mills with approximately 500 g zirconium media were used.A mixture of 2.5 g pigment and 25 g of “base 2” white paint was allowedto roll for 1 hour. The mixture was pipetted onto a Leneta opacitychart, where the pigment mixture was spread with a #52 stainless steelrod. Once the layered pigment mixture was dry it was analyzed using aDatacolor™ 450 machine. DataColor™ results shown in table 12 are theactual L* a* b* parameters of the precipitate color.

Synthetic iron oxide pigments are produced commercially in a range ofcolour shades from “yellow-shade” red pigments to “blue-shade” redpigments. Analysis data for Bayferrox™ 105M and 130M (Lanxess) have beenincluded in Tables 11 and 12 for reference purposes. These pigments aretwo of the more commonly used synthetic iron oxide products presentlyavailable. The PPT samples shown in Table 12 have colour parametersfalling into the same general range as Bayferrox™ 105M and 130M.

Samples for particle size analysis were prepared using about 2.5 gramsof pigment solids added to 80 ml of 50% glycerin. The pigment andglycerin were then allowed to mix in a beaker using a magnetic stirrer.After 5-10 minutes the mixture was sonicated with 20% power at anamplitude of 40. Once the agglomerates were broken the pigment/glycerinmixture was added to the mixing chamber of the Sedigraph 5100™ analyzer.

The tests, summarized in Tables 11 and 12, demonstrate the ability tocontrol the particle size and precipitate colour characteristics byvarying the seeding ratio. Reducing the seeding ratio from 300 to 400%down to 50% increased the precipitate particle sizes (d50) from 0.37microns up to greater than 1.34 microns. This size range ischaracteristic of pigment grade synthetic iron oxide materials that areconventionally produced via a Penniman-style process. The use of higherprecipitation temperature was also identified as a method to furtherincrease particle size.

As the seeding ratio is reduced and particle size is increased, thecolour shade of the precipitates is gradually changed from a“yellow-shade” red (PPT 18 and PPT 20) to a “blue-shade” red (PPT 22).The colour shift can be seen in Table 12 as the “a” and “b” parameterschange from values similar to Bayferrox™ 105M at high seed ratios (PPT18 and PPT 20) to those similar to Bayferrox™ 130M at low seed ratios(PPT 22). This ability to produce a range of pigment-grade shades allowsthe process to be a viable alternative to more conventional techniques.

A series of additional tests were performed to determine the impact ofprocess parameters on the precipitation of iron oxide solids at elevatedtemperatures/pressures. Some process parameters that were examinedincluded precipitation temperature, concentration of iron and freenitric acid in the precipitation feed solutions and reaction time. Theresults of these tests are summarized in Tables 13 to 16. The equipmentand procedures for these tests were identical to those outlined for thetests whose results are summarized in Table 11.

TABLE 13 Effect of temperature on precipitation of iron from solution.Initial Iron Seed Initial Iron free precipitation Temp Ratioconcentration acid Reaction from Test (° C.) (%) (g/L) (g/L) Time (hr)sol'n (%) PPT 4 150 300 49.8 32 1 40.8 PPT 5 175 300 49.8 32 1 63.1 PPT6 200 300 49.8 32 1 75.5 PPT 20 225 300 49.8 32 1 87.5

TABLE 14 Effect of solution iron concentration on precipitation of ironfrom solution. Seed Temp Ratio Initial Fe Initial FA Time Feprecipitation Test (° C.) (%) (g/L) (g/L) (hr) from sol'n (%) PPT 11 175300 38 32 1 70.3 PPT 10 175 300 49.8 32 1 61.6 PPT 12 175 300 54.8 33 157.7

TABLE 15 Effect of solution free acid concentration on precipitation ofiron from solution. Seed Temp Ratio Initial Fe Initial FA Time Feprecipitation Test (° C.) (%) (g/L) (g/L) (hr) from sol'n (%) PPT 10 175300 49.8 33 1 61.6 PPT 13 175 300 46.0 61 1 55.9

TABLE 16 Effect of reaction time on precipitation from or iron fromsolution. Seed Temp Ratio Initial Fe Initial FA Time Fe precipitationTest (° C.) (%) (g/L) (g/L) (hr) from sol'n (%) PPT 4 150 300 49.8 32 140.8 PPT 14 150 300 49.8 32 0.5 33.9 PPT 5 175 300 49.8 32 1 63.1 PPT 15175 300 49.8 32 0.5 55.6 PPT 6 200 300 49.8 32 1 75.5 PPT 16 200 30049.8 32 0.5 72.1

The CIELAB colour analyses also demonstrate that the colour intensity(“L” parameter) is similar to that of the Bayferrox™ 105M/130Mstandards. Colour intensity is affected by the size distribution of thepigment materials. The presence of a wide distribution with largerquantities of fine particles results in poorer colour strength (reduced“L” intensity parameter). Table 17 below shows the slope factors of theparticle size distributions for the pressure precipitated samples versusthose of the Bayferrox™ 105M/130M standards. The distribution slopefactors were calculated as 1/(d₈₀-d₅₀). Particle diameters indicated asd₈₀ are the size at which 80% of the solids are finer than. A steeperslope (larger value of 1/(d₈₀-d₅₀)) indicates a narrower sizedistribution and therefore less fine. The pressure precipitated sampleshave slope values and colour intensities (“L” parameter) typical (orbetter) than the range of synthetic iron oxide pigments represented bythe Bayferrox™ 105M/130M standards.

TABLE 17 Slope of precipitated iron oxide size distribution curves.Slope Test 1/(d80 − d50) PAL22 0.66 PAL27 1.24 PAL23 2.60 PAL1 2.30 PAL72.86 PAL8 3.75 PAL18 3.79 PAL20 3.75 Bayer 105 2.79 Bayer 130 0.78

The data summarized in Tables 13 to 17 provides the following insightsinto the elevated pressure precipitation reactions. Increasing theprecipitation temperature shifts the equilibrium point for the ironprecipitation reactions and improves the recovery of iron from solutionto the iron oxide solids. Increased levels of dissolved iron and freenitric acid in solution shift the equilibrium of the precipitationreactions in the opposite direction and therefore result in reductionsin the recovery of iron to the precipitated solids. The impact ofsolution concentration changes is greatest for the dissolved iron levelsdue to the fact that each mole of dissolved iron generates 3 moles offree nitric acid when the iron is precipitated.

At lower precipitation temperatures (150-175° C.) there is a noticeableimprovement in iron precipitation recovery when the reaction time isincreased from 0.5 to 1.0 hours. As the precipitation temperature isincreased (200° C.) the reaction kinetics increase to the point that theprecipitation reactions are essentially complete after 0.5 hours.Further increases in temperature should allow for additional reactiontime reductions.

Example 7 Simulated Continuous Cycle for Seeded Process

Simulated continuous tests or locked-cycle experiments were performed.The leach solution for these tests was prepared by dissolving amagnetite concentrate (Iron Ore Company of Canada) in 25% nitric acidsolution. The resulting solution was then diluted with distilled waterto produce a final solution having a dissolved iron content of 42-45g/L. The resulting diluted solution contained approximately 45 g/L offree nitric acid.

The locked-cycle tests were used to simulate a continuous process viathe use of a series of batch reactions. The same apparatus describedabove for the batch tests was used for the locked-cycle work. For theinitial test in each cycle, Bayferrox™ 105M (Lanxess) was used as theinitial seed material. After the first precipitation is complete, theprecipitated solids are removed. At that time a portion of these solidsis then separated and used as the seed material for the secondprecipitation in the series. This process is repeated a number of timeswith the same seeding ratio maintained throughout the test series. Thetotal number of tests in the cycle is selected to ensure that theprecipitated pigment solids from the final test is representative ofwhat would be expected from a continuously operating process. Only thesolids from each test are recycled to the next test in the series. Thetest filtrates are analyzed and then discarded.

After several repetitions (typically 5-10), the original Bayferrox™ 105M(Lanxess) solids used as seed in the first precipitation are essentiallyremoved from the system and replaced with novel precipitate product. Thehigher the seed ratio that is used, the more repetitions required in theseries in order to effectively eliminate the original seed material. Theproducts from each test (solids/liquids) were analyzed according to thesame procedures as were applied in the batch test procedures. Theseresults are shown in Tables 18 and 19.

TABLE 18 Summary of continuous results. Temp Seed Initial Fe Initial FARepetitions Test (° C.) (%) (g/L) (g/L) in test series Locked Cycle 1200 200 42 45 5 Locked Cycle 2 200 80 42 45 5 Locked Cycle 3 200 350 4545 8

TABLE 19 A summary of the colour analysis and size of the precipitatesobtained from the “locked-cycle” test series in table 18. d₅₀ Test L* a*b* (microns) Locked Cycle 1 49.87 25.14 15.56 0.96 Locked Cycle 2 54.0020.01 12.50 4.5 Locked Cycle 3 50.71 25.88 16.68 0.90 Bayferrox ™ 53.1529.05 24.01 0.60 105M Bayferrox ™ 50.67 25.97 16.46 0.70 130M

The locked-cycle test series were performed at 200° C. with the seedratio varying from 80% to 350%. Table 19 shows the size and colouranalyses for the final precipitate produced in each test series. Onceagain, Bayferrox™ 105m/130m (Lanxess) pigments were included forreference. The recovery of iron from solution into the precipitatedsolids varied from approximately 80-90% for all of the individual testsin each locked-cycle test series.

The precipitates produced by the locked-cycle tests series usingmagnetite leach solutions were coarser than those produced from thesingle batch leach tests when similar precipitation parameters wereutilized. The simulated continuous reaction results also illustrate thefact that particle size and colour parameters can be controlled byvarying the seed ratio to produce pigment-grade precipitate solids. Thehighest seed ratio used in the locked-cycle tests produced the finestprecipitate (d₅₀=0.90 microns). As the seed ratio was reduced, theprecipitate d₅₀ particle size increased (0.96 at 200% seed and 4.5microns at 80% seed). The precipitate size changes were accompanied by acorresponding colour shift between more “yellow-red” materials at highseed ratios (finer particles) and more “blue-red” at low seed ratios(coarser particles). These trends are identical to those shown in thebatch process.

As understood by those skilled in the art, additional embodiments may bepracticed within the scope and intent of the present disclosure of theinvention. The embodiments above are intended to be illustrative and notlimiting. Additional embodiments are within the claims. Although thepresent invention has been described with reference to particularembodiments, workers skilled in the art will recognize that changes maybe made in form and detail without departing from the spirit and scopeof the invention.

1. A process for the production of ferric oxide precipitates having aparticle size from about 0.1 to about 10 microns, the process comprising(a) obtaining an aqueous feed solution comprising iron solubilized inone member selected from the group consisting of nitric acid, sulfuricacid, and hydrochloric acid, the aqueous feed solution having a pHranging from about 0.25 to about 2.5; and (b) subjecting the aqueousfeed solution to a combination of (i) a temperature from about 100° C.to about 300° C., (ii) a seeding ratio from about 20% to about 2000%,wherein the seeding ratio is a ratio of a weight of a seed solid to aweight of an expected unseeded precipitate product, and wherein theparticle size of the ferric oxide precipitates is smaller than aparticle size of the ferric oxide precipitates obtained with a seedingratio of 0%, and pressures ranging from about 40 psig to about 1300 psigto obtain ferric oxide precipitates of the particle size from about 0.1to about 10 microns.
 2. The process of claim 1, wherein the temperatureis from about 175° C. to about 240° C.
 3. The process of claim 1,wherein the seeding ratio is from about 50% to about 500%.
 4. Theprocess of claim 1, wherein the selected particle size is from about0.15 to about 2.5 microns.
 5. The process of claim 1, wherein the ferricoxide precipitates are obtained in from about one minute to about 6hours.
 6. The process of claim 1, wherein the ferric oxide precipitatesare obtained in from about 30 minutes to about 1 hour.
 7. The process ofclaim 1 wherein said process is conducted at a pressure of from about100 to about 500 psig.
 8. The process of claim 1, wherein the ferricoxide precipitates are obtained from a feed solution comprising ironsolubilized in nitric acid.
 9. The process of claim 1, wherein the feedsolution has an iron concentration of from about 5 g/L up to the onsetof crystallization of a ferric salt.
 10. The process of claim 1, whereinthe feed solution has an iron concentration of from about 10 g/L toabout 100 g/L.
 11. The process of claim 1, wherein the feed solution hasan iron concentration of from about 30 g/L to about 60 g/L.
 12. Theprocess of claim 1, wherein the feed solution has a free acidconcentration of from about 5 g/L to about 150 g/L.
 13. The process ofclaim 1, wherein the feed solution has a free acid concentration of fromabout 30 g/L to about 70 g/L.
 14. The process of claim 1, wherein theferric oxide precipitates have an L* of about 40 to about
 60. 15. Theprocess of claim 1, wherein the ferric oxide precipitates have an L* ofabout 49 to about
 55. 16. The process of claim 1, wherein the ferricoxide precipitates have an a* of about 10 to about
 40. 17. The processof claim 1, wherein the ferric oxide precipitates have an a* of about 19to about
 33. 18. The process of claim 1, wherein the ferric oxideprecipitates have an b* of about 5 to about
 35. 19. The process of claim1, wherein the ferric oxide precipitates have an b* of about 12 to about28.
 20. The process of claim 1, conducted in a batch or a continuousfashion.
 21. The process of claim 1, wherein the ferric oxideprecipitates have a smooth surface texture.